Vladimir Putin says he has sent his condolences to the family of Yevgeny Prigozhin after the Wagner Group leader was presumed killed in a plane crash.
The Russian president said Prigozhin was a talented businessman who he had known since the 1990s.
This is the first time Mr Putin has commented on the plane crash, which happened north of Moscow yesterday and killed all 10 people on board.
If confirmed, Prigozhin's death would leave Mr Putin stronger in the short term - removing a powerful figure who had played an instrumental role in the invasion of Ukraine, and criticised Russia's top military officials.
The mercenary chief had led a brief rebellion in June - demanding the ousting of defence minister Sergei Shoigu as he marched his fighters towards Moscow before calling off their advance.
Prigozhin's right-hand man Dmitry Utkin was also believed to be on board, alongside his security chief Valeriy Chekalov and other lieutenants.
It is unclear why so many high-ranking members of Wagner, who are normally exceedingly careful about their security, were on the same flight.
In televised comments, Mr Putin said they had made "a significant contribution" during the Ukraine war that will not be forgotten.
The president described Prigozhin as "a person with a complicated background who made serious mistakes in his life" - but noted that he was also a successful entrepreneur.
According to Mr Putin, Prigozhin had returned from Africa only yesterday - and had then gone on to meet Russian officials in Moscow.
A preliminary investigation into the crash has now begun, with Mr Putin expecting that further details will emerge "soon".
Unconfirmed Russian media reports have claimed that bodies at the crash site are burned and disfigured beyond recognition - meaning they would need to be identified by DNA. Vehicles have been taking away the bodies.
The president has been widely accused of having involvement in the crash - with some Western experts believing Mr Putin may have ordered Prigozhin's assassination. This cannot be verified.
Two US officials, speaking on the condition of anonymity, have said they believe a surface-to-air missile originating from inside Russia likely shot the aircraft down.
Pro-Wagner channels on the Telegram messaging app have also claimed that the jet was deliberately downed - with some suggesting that a bomb could have been placed onboard.
A makeshift memorial has been built by Prigozhin's supporters at Wagner's headquarters in St Petersburg.
Sergei Mironov, who leads the pro-Kremlin Fair Russia party, posted on social media: "Prigozhin messed with too many people in Russia, Ukraine and the West. It now seems that at some point his number of enemies reached a critical point."
Several of Mr Putin's critics and opponents have been killed or gravely hurt in apparent assassination attempts - with German foreign minister Annalena Baerbock saying it is "no coincidence" that fingers are being pointed at the Kremlin.
"We know this pattern... in Putin's Russia - deaths and dubious suicides, falls from windows that all ultimately remain unexplained," she added.
Volodymyr Zelenskyy has denied that Ukraine was involved in the plane crash, adding: "We have nothing to do with this. Everyone understands who does."